1. Field of the Invention
The present invention relates to the field of image processing and the field of digital camera application, and more particularly to an original image dynamic range compression method, an apparatus, and a digital camera, in which the dynamic range of an input image is mapped onto the dynamic range of a reproduction medium to reproduce the image in the medium.
2. Description of the Related Art
In the field of image processing and the field of digital camera application, dynamic range is also referred to as a brightness range, which is expressed by the ratio of the highest brightness and the lowest brightness in a scene or an image. For example, the dynamic range of an image pickup device such as a digital camera or a display device such as a personal computer display, is far smaller than that of the natural scene. Therefore, in order to display everything in the scene, it is necessary to apply dynamic compression or a conversion technique referred to as tone mapping.
In the field of cameras, methods for implementing tone mapping have already been developed. These methods can be classified into total range methods and local methods. The total range method is performed by separately processing each pixel in an image. The total range method takes into consideration the overall characteristics of an image. The total range method is typically performed by calculating a monotone mapping curve, and mapping each pixel of the input image onto the output image based on the calculated curve. The local method is typically performed by imitating the characteristics of a visual system of a human being. For example, it is rationally hypothesized that a human being's eyesight is adjusted based on the local characteristics of a target point, instead of on the overall image. For each pixel in an input image, the value in the output image is determined based on local image characteristics near the corresponding pixel. The calculating speed of the total range method is higher than that of the local method, and real-time calculation can be achieved with the use of a computer. However, with the total range method, detailed portions may be lost from a scene having a considerably large input dynamic range. The local method is more advantageous in terms of mapping detailed portions of a scene, but the calculating speed is lower than the total range method, and a defective image may be formed (for example, a blurred image).
In the tone mapping of the related art, different parameters are used for different regions of interest (ROI). There is an eyeball tracing method and a scan route tracing method for finding the region corresponding to the user's region of interest. The eyeball tracing method is performed by tracing the movement of the eyeball of the photographer. However, when different tone mapping parameters are used for different regions, discontinuous luminance change may occur at the boundaries between the regions. Furthermore, it is not easy to detect the region of interest. If there is an error in finding the region, the entire image may not be successfully converted. Furthermore, additional devices are required for performing the eyeball tracing method, which increases the cost of the digital camera.
There is proposed another tone mapping method in the related art. Specifically, a small difference between contrast ratios of different regions in an image is retained, but a large difference between contrast ratios of different regions in the image is compressed. For example, a hard threshold of 4:1 is specified for the local contrast ratio. When the local contrast ratio exceeds this hard threshold, the corresponding local contrast ratio is reduced (compressed) to 4:1. However, when an image has significantly large local contrast ratios, the image quality may be degraded by performing this compression method with the use of a hard threshold. Furthermore, it is time-consuming to calculate local contrast ratios. Accordingly, application of this method has limitations.
There is a large difference between the dynamic range in an image pickup device or a display device, and the dynamic range of a natural scene. Therefore, when a natural scene is reproduced in a digital camera or a computer display, it is necessary to compress the dynamic range of the natural scene by the tone mapping technology. Furthermore, with the advancement in the digital camera technology, image resolution is becoming increasingly high. In conceiving the present invention, the following findings were obtained. That is, currently available digital cameras can take an image of 10 million pixels, but the processing capacities of the CPU and the memory of a digital camera are lower than that of a computer. Accordingly, when applying the dynamic range compression technology to a digital camera, it is necessary to increase the speed of the tone mapping process, and to solve the problem of the low processing capacity of the digital camera in processing large (high-resolution) images.